1. Field of the Invention
The present invention generally relates to a printing press and more specifically to an offset printing press having: a frame; a plate cylinder being rotatably mounted on the frame; an inking unit for supplying ink to the plate cylinder. The inking unit comprises: an ink fountain for containing ink; a plurality of inking rollers; means for transferring ink between said ink fountain and said plurality of inking rollers; a plurality of ink applicator rollers for transferring ink between said plurality of inking rollers and said plate cylinder. The printing press further includes a dampening unit for supplying damping medium to said plate cylinder. The dampening unit comprises: a forme roller having an elastic outer cylindrical surface, the form roller being engageable at a printing plate of a plate cylinder; a driven dampening distributor having a chromium outer cylindrical surface and being assigned to the forme roller; and further dampening-unit rollers supplying the forme roller with the dampening medium contained in a dampening-medium vessel.
2. Background Information
An arrangement such as that described above is known from German Patent No. 31 46 223 C2, corresponding to U.S. Pat. No. 4,440,081 (Apr. 3, 1984), in which an additional dampening distributor having a rough outer cylindrical surface intensively works the dampening water, or damping medium, into the ink film disposed therebeneath in order to form an emulsion. With this known embodiment, the dampening forme roller always essentially rotates at the circumferential speed of the plate cylinder.
If a forme roller is to be driven at a lower speed than the plate cylinder so as to eliminate dirt particles from the plate surface, it is known to fasten a gear rim, possibly on the front side of the plate cylinder between the machine side frames. Such a gear rim would essentially transmit the drive of the forme roller via a gearwheel provided on the front side of the forme roller. Alternatively, the forme roller can be driven via a separate motor which ensures that the circumferential speed of the forme roller is lower than the circumferential speed of the plate cylinder (German Patent No. 18 08 909 C3). Such drives can have the disadvantage that the gearwheels are not sufficiently provided with lubricating oil, which leads to a higher degree of wear and tear. Moreover, there is the risk of the print product being soiled by lubricating oil. Furthermore, it has appeared that the gear drive of the forme roller may cause so-called "gear marks", which would mean that the ink and/or the dampening medium is not uniformly transferred onto the print product. As a result thereof, marks extending transversely to the print direction would be seen. This effect can potentially produce waste paper.